1. Field of Invention
The present invention relates to a method for manufacturing liquid crystal display panels, and especially a method for manufacturing thin film transistor (TFT) panels.
2. Related Art
The TFT liquid crystal display panel is a very popular planar display panel with the advantages of small volume, high resolution and low power consumption. It has been widely used in information display devices of various electronic products, such as TVs  computers  cellular phones  personal digital assistants (PDA)  liquid crystal projectors and information appliances (IA). The thin film liquid crystal display panel (TFT-LCD) contains two separated parallel glass panels. Liquid crystal material is filled into the space between the two separated parallel glass panels. An active-matrix driving circuit of thin film transistors is usually formed on one of the separated parallel glass panels. Each of the thin film transistors corresponding to one specific pixel of the liquid crystal display panel is used to turn on/off the pixel. The glass panel with the thin film transistor driving circuit can be called a thin film transistor panel. According to the structural differences of the thin film material, the thin film transistor panel can be classified as an amorphous silicon (a-Si) thin film transistor panel or a polysilicon (p-Si) thin film transistor panel. Due to defects in the amorphous silicon, the mean free path of electrons is influenced and electron mobility is reduced. The electron mobility with polysilicon is one hundred times greater than the electron mobility with amorphous silicon. Therefore, the application of polysilicon thin film transistors on the driving circuit of the display has been accepted by the thin film transistor display industry. FIG. 6 is a cross-sectional view of a polycrystal thin film transistor. The method of manufacturing polycrystal thin film transistors involves forming a layer of polycrystal silicon 2. The polycrystal silicon layer comprises two heavily doped n+ regions 3  4, which are source 5 and drain 6 respectively. One insulator 7 is formed on the glass substrate 1 and the polycrystal layer 2. However, the two heavily doped n+ regions 3  4 are exposed in order to form a contact via connecting the source 5 and drain 6 later. A gate 8 is formed between the two heavily doped n+ regions 3  4, and above the polycrystal silicon layer 2. A gate oxide layer 9 is formed between the gate 8 and the polycrystal silicon layer 2. Conventionally, high temperature processes cannot be used to form a polycrystal layer providing high electron mobility on the glass substrate because the tolerable temperature of the glass substrate 1 is lower than the processing temperature. Laser annealing is a method often used for forming the polycrystal silicon layer 2. The laser annealing process involves forming a layer of amorphous silicon on the glass substrate by chemical vapor deposition. Then the amorphous silicon layer is recrystallized to form a polycrystal silicon layer by excimer laser annealing (ELA). However, excimer laser annealing is not a very stable process. It is also very difficult to form a high quality crystal thin film on an amorphous glass substrate. The polycrystal silicon layer formed by the laser annealing process is thus not very uniform. Its surface is too rough and causes current leakage of the gate oxide. When this happens, the transistor is damaged and the reliability of the thin film transistor panel is negatively affected.
Therefore, It is necessary to have a novel method for manufacturing thin film transistor panels that provides high performance and reliability without the problems mentioned above.
The purpose of the invention is to provide a novel method for manufacturing thin film transistor panels to overcome the low stability of the conventional laser annealing process and resultant low quality of the polycrystal silicon thin film.
According to purposes mentioned above, the method of the invention for manufacturing thin film transistor panel is to form a transparent insulator on a silicon substrate. Form a thin film transistor structure and transparent electrode on the transparent insulator. A high temperature process can be used to obtain a thin film transistor with high uniformity and high electron mobility. The thin film transistor is, for example, high quality polycrystal silicon thin film. After that, a transparent substrate is bonded onto the surface of the silicon substrate. The back of the silicon substrate is then polished by chemical mechanical polishing, or etched to remove the silicon substrate. A transparent thin film transistor is thus formed by this method. This method comprises the following steps: provide a silicon substrate; form a layer of transparent insulation on the front surface of the silicon substrate; form a plurality of thin film transistor structures and a plurality of corresponding transparent electrodes on the transparent insulator; bond a transparent substrate onto the front surface of the silicon substrate; remove the silicon substrate; and expose the transparent electrode by etching the transparent insulator.
According to the invention, a color filter can also be added into the manufacturing process if necessary. For example, the color filter can be formed on the thin film transistor structure layer. An alignment mark can also be formed on the transparent insulator so as to be exposed from the transparent insulator side (the alignment mark can be seen from the bottom surface side) after the silicon substrate is removed. Therefore, it can be of help if the transparent insulator side undergoes a subsequent photolithographic process.
Before bonding the transparent substrate in the invention, a black matrix can be formed on the thin film transistor structure layer. The black matrix defines the transparent range of pixels on the thin film transistor panel and masks the portion of the thin film transistor panel that is most likely to incur light-leakage problems. The black matrix can also be a photo mask in the step of etching the transparent insulator for exposing the transparent electrode. In this case, a back exposure process is used (expose from the transparent substrate side of the thin film transistor panel). A positive photo-resist on the bottom of the transparent insulator will be exposed without an additional photo mask. A subsequent photolithographic process can be performed to generate a suitable contact via on the transparent insulator. The contact via allows the transparent electrode to be exposed from the transparent insulator side of the thin film transistor panel. Hence, this traditionally complicated process is simplified, and productivity is increased.
According to the invention, the transparent electrode can also be formed on the bottom surface of the transparent insulator. This method comprises the following steps: provide a silicon substrate; form a transparent insulator on the silicon substrate; form a plurality of thin film transistors on the surface of the transparent insulator; bond a transparent substrate onto the front surface of the silicon substrate; remove the silicon substrate; and form a plurality of transparent electrodes corresponding to the thin film transistor structures on the bottom surface of the transparent insulator.
According to method of the invention: A color filter can be formed on the bottom surface of the transparent insulator. A black matrix can be formed on the thin film transistor structure if required.
According to purpose of the invention, another method for manufacturing thin film transistor panels is to bond a transparent substrate onto the back of the silicon substrate, and reduce the thickness of the silicon substrate to form a layer of crystal silicon thin film. Form the required thin film transistor structure layer and transparent electrode on the crystal silicon thin film. This method comprises the following steps: provide a silicon substrate; bond a transparent substrate onto the back of the silicon substrate; reduce the thickness of the silicon substrate to form a layer of crystal silicon thin film; Form a plurality of thin film transistor structures on the crystal silicon thin film; etch the thin film transistor structure layer and the crystal silicon thin film to form suitable pixel via; form a planarization layer on the thin film transistor structure and the pixel via; and form a plurality of transparent electrodes corresponding to the thin film transistor structures on the planarization layer.
The process for reducing the thickness of the silicon substrate can be wafer polishing or etching. The planarization layer can be produced by color filter material. In this case, the planarization layer also provides the function of color filtering.
The method of the invention for manufacturing thin film transistor panels solves the traditional problems of forming thin film transistors on glass substrate and provides advantages such as high stability, high aspect ration and high productivity.
Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.